Polychlorinated biphenyls are a class of 209 congeners with varying biological actions. Humans are exposed to many of these; in human tissue samples, depending on the chemical methods used, the number of congeners that can be detected ranges as high as 60 or 70. In addition, humans are exposed to related compounds, such as dioxins and furans. When studying health effects of PCBs, the appropriate exposure measure to use is not clear; most studies either have used a measure of total PCBs or have examined selected individual congeners. In deciding what measure to use, it is important to realize that exposures of humans are to mixtures of relatively standard proportions rather than to individual congeners independently. The resulting correlations among congeners impact the conclusions that can and should be drawn from studies of health effects, so it is important to know what the correlations usually are. We have described correlations among 34 PCBs, 7 dioxins, and 5 furans in blood from 63 Canadian blood donors. We are currently examining correlations among 40 PCBs in milk from 497 Canadian mothers.